1. Field of the Invention
The present invention generally relates to wireless communication systems and, more particularly, to base station transceiver subsystems used in a Code Division Multiple Access (CDMA) network or other digital and analog telecommunication systems.
2. Description of Related Art
FIG. 1 (prior art) is a block-flow diagram which graphically represents a wireless communication system. From FIG. 1 it is seen that a basic wireless communication system comprises a mobile station 10, a base station 20, a reverse link 30, which represents the electromagnetic wave communication link transmitted from mobile station 10 to base station 20, and a forward link 40 which represents the electromagnetic wave communication link transmitted from base station 20 to mobile station 10.
FIG. 2 (prior art) shows a cell grid and cell sites. In a wireless communication system based on the general cellular principle, a service area 49 is divided geographically, into a number of small areas 50, 52, 54, 56 called xe2x80x9ccells.xe2x80x9d In each cell there is a cell site 58, 60, 62, 64 where radio equipment known as a Base Station Transceiver Subsystem (BTS) is installed. Multiple cell layouts such as macro cells, micro cells, and Pico cells can be provided within a particular geographical area to effect hierarchical coverage (where macro cells provide the largest coverage and Pico cells the smallest). Pico cells may be used to provide coverage inside buildings, to cover a special area (campus, stadium, airport and shopping mall), to temporarily cover for special events or areas hit by natural disasters, to cover outlying remote locations, to supplement macro or mini cells with hole-filling, or to enhance the capacity of hot spots. FIG. 3 (prior art) is a block diagram of a wireless system network connected to a land line Public Switched Telephone Network (PSTN) 68. As shown in FIG. 3, a BTS 66 provides a link to mobile subscribers or (mobile stations) 10. Each BTS 66 typically may include two or more antennas 67, which may be omni antennas or directional antennas. Omni antenna configurations provide 360xc2x0 of coverage, whereas directional antennas provide less than 360xc2x0 of coverage across an area known as a sector. For example, there may be two, three or more sectors in a typical directional configuration such that each sector of a two sector configuration generally provides 180xc2x0 of coverage and each sector of a three sector configuration generally provides 120xc2x0 of coverage, etc. For satisfactory reception and transmission, each sector typically requires at least two antennas for diversity reception.
Continuing with the description of FIG. 3, each BTS 66 is coupled to a Base Station Controller (BSC) 70 (multiple BTSs 66 may be coupled to a single BSC 70). Likewise, each BSC 70 is coupled to a Mobile Switching Center (MSC) 72 and the MSC 72 is in turn coupled to a PSTN 68.
FIG. 4 (prior art) is a functional block diagram of a BTS. As shown in FIG. 4, a conventional BTS 66 typically comprises four major functional blocks for each sector of coverage: an RF front-end 74, a plurality of transceivers 76, a plurality of modem processors 78, and a controller 80. Controller 80 interfaces with a BSC 70 over a T1 or E1 line 81, and the RF front-end 74 is connected to the antennas 67 which are typically mounted at the top of a tower or pole 82 as represented in FIG. 5 (prior art), where FIG. 5 illustrates an outdoor and ground based BTS coupled to a tower topped mounted antenna.
In a typical system, the four major functional blocks of the BTS 66, shown in FIG. 4, are contained in one physical cabinet or housing which is in close proximity to a pole (or tower) 82 at ground level. Long coaxial cables 84 are then run to the top of the pole 82 where the antennas 67 are mounted. The cable length typically varies from 50 to 200 feet, depending on various installation scenarios. Cables of these lengths suffer from undesirable power losses. Accordingly, thick coaxial cable diameters of approximately xc2xe to 1xc2xd inches are used to minimize the cable power loss, which is typically about 2 to 4 dB. Minimizing these power losses is important because such losses in the cables degrade the receiver sensitivity and reduce transmission power.
FIG. 5 depicts a prior art BTS unit 66 connected via a long length of cable 84 to an antenna 67 at the top of a supporting structure 82. FIG. 6 (prior art) is a block diagram of yet another known BTS architecture where a tower top mounted RF front-end module consists of a Low Noise Amp (LNA) and a Power Amp (PA) 74 (hereinafter LNA/PA unit 74). The cable power loss in this architecture is not as critical as in the previous mentioned architecture because the power loss can be made up with additional amplification. However, there is still a need to use rather thick cables due to the signals between the LNA/PA unit 74 and the transceiver 76 in the BTS 66 are high frequency/radio-frequency (RF) signals. Other problems are associated with transmitted RF signals between the LNA/PA unit 74 and the BTS 66, such as power losses, system noise, and mechanical clutter. Furthermore additional complex circuitry either or both in the RF front-end module and the transceiver may be required to automatically compensate for the wide range of cable losses that arise in different installation scenarios due to varying cable lengths. Such problems get more severe as the operating RF Frequencies are allocated in the increasingly higher frequency bands. This is the case for personal communications systems.
In other words, as the length of a cable 84 increases, or as the frequency transmitted through a cable 84 increases, power losses between the LNA/PA unit 74 and the ETS 66 increase. Thus, the long cables 84 used to connect the LNA/PA unit 74 to the BTSs 66 (often in excess of 150 feet, sometimes even exceeding 300 feet) introduce large power losses. For example, a 100 W power amplifier in a base station transceiver unit transmits only 50 W of power at the antenna when there is a 3 dB loss in the cable. Power losses in the cable work against reception as well, reducing the ability of the receiver to detect received signals. Also, with Personal Communication Systems (PCS) operating at high frequencies, the power loss in the cable 84 running between the LNA/PA unit 74 and the transceiver 76 in the BTS 66 increases. Thus, RF cable losses incurred on both the transmit and receive paths result in poorer than desired transmission efficiency and lower than desired receiver sensitivity, making the use of relatively thick (high conductance) coaxial cables necessary to minimize loss.
Generally, in a wireless environment, wherein radio frequencies are transmitted through air, interferences are inevitable. That is, unless a transmitting antenna is directly in the line-of-site of the receiving antenna and no obstacles, such as trees, buildings, rock formations, water towers, etc., are in the way, then reflections will cause fading and multipath signals. In order to minimize the effects of fading and multipath, diversity receivers can be used increase the carrier-to-noise ratio (and/or Eb/No. A diversity receiver requires its own antenna. Thus, for each transmission frequency two antennas are used on the receiving side. One antenna is a transmit/receive antenna and the second antenna is used for a diversity receiver which is utilized to overcome some of the fading and multi-path problems.
In some cell sites where the communication capacity is high, there is a need to transmit more than one RF carrier signal. The transmission of multiple RF carriers per sector requires a corresponding number of transmit antennas per sector. Additional receiving antennas are also required especially if diversity receivers are utilized in the system. Increasing the number of antennas creates an xe2x80x9ceye-sorexe2x80x9d for the public and is not desirable.
A conventional technique for reducing the number of transmit antennas required for multiple RF carrier transmission are shown in FIGS. 7 and 8.
In FIG. 7 (prior art) the carriers are combined with a high power combiner. In FIG. 8 (prior art) the carriers are combined at low power and then the combined signal is amplified with a multi-carrier power amplifier.
Neither design is suitable for use in a compact BTS system due to high power loss in the combiners and the inability to provide diversity reception.
What is needed is a compact BTS system that can be adapted to handle multiple transmit and receive frequencies, multiple sector configurations, multiple wireless communication protocols, and be able to transmit signals at a variety of power levels for different types of cells (eg. macro-, micro-, pico-) without increasing the number of antennas significantly or substantially decreasing the overall performance of the system.
The present invention provides a BTS wherein a radio unit (RU) is located proximate to the antenna mounting location. A main unit (MU) is connected to and remotely located from the RU. One or more antennas are coupled to the RU. There can be a plurality of RUs connected to a single MU. The plurality of RUs may operate on the same or different frequencies, the same or different transmit power, the same or different wireless communication protocols.
An object of the exemplary embodiment of the present invention minimizes the number of antennas required for multiple frequency, multiple communication protocol, or variable transmit power BTS system.
Another object of the present exemplary BTS system allows for two RUs to be connected together to thereby increase the number of operating frequencies, or communication protocols, while maintaining transmission power level without increasing the number of antennas.
Another object of the present invention is to increase call capacity of a BTS without increasing the number of antennas for a cell, thereby minimizing the cost of increasing the call capacity.
Another object of the present invention is to transmit and receive two frequencies or wireless protocols with two antennas and maintain diversity reception. The diversity receiver helps to minimize the effect of fading and multipath.
There are many advantages to this exemplary architecture and some of them are as follows:
A compact size RU is provided which can be easily mounted close to the antennas, whereby cable loss is virtually eliminated. Cable losses degrade the receiver sensitivity and reduce the transmit power. The present invention, thus, allows for a relatively low power PA and provides a transmit power level equivalent to a higher power PA used in a prior art BTS.
The inclusion of the transceiver in the RU allows for a lower frequency interface rather than an RF interface typically used in prior arts, to the MU. The lower frequency interfaces yield lower cable losses, thus allowing the use of inexpensive and small diameter interconnect cables between the RUs and the MU.
The separation of RF elements and dependent elements thereof, also, result in easier adaption of the BTS design to support different RF operating environments or conditions, as in different frequency bands and different transmission power levels, as only the RU needs to be modified, while the same MU is used. This also results in a compact size MU for ease of handling and mounting. This is because less space and weight are required without RF elements installed and, at the same time, less heat is generated in the MU requiring cooling.
This architecture allows a wireless communication provider to provide service via a variety of wireless protocols without the need for a different BTS for each protocol.
This architecture also allows the BTS to be configured to support either omni or sector operations, or to upgrade from omni to sector operations as the traffic demand goes up. This is especially important in CDMA systems where softer handoffs need to be supported between the sectors. For an omni configuration, only one RU is needed. For two or three sector configurations, two and three RUs are needed, respectively. The three RUs can be operated on the same frequency in a three sector configuration or at different frequencies in a three carrier omni configuration.
The present invention also allows the connectivity of another set of three RUs connected to its own MU to the same antennas without the use of a combiner.
By locating the transceiver module in the RU, only low frequency signals need be passed from the transceiver module and the MU. On the receive side, the transceiver module converts a high frequency signal to a low frequency signal, and on the transmit side, the transceiver module converts a low frequency signal from the MU to a high frequency signal for transmission. Thus, only low frequency signals are passed between the RU and MU, minimizing power loss in the cables connecting the two units. This results in the ability to use smaller diameter, less costly cables.
Another advantage to removing the transceiver subsystem from the MU is that the resulting MU is physically much smaller in size and weighs less. This translates into easier installation and maintenance, as well as into flexibility in meeting the technical demands of a challenging operating assignment or challenging environmental considerations. In addition, smaller size and lighter weight BTSs are especially advantageous for Pico-cell applications or micro-cell applications where a greater number of BTSs are required than are needed for macro cell implementations.
Since the entire transmit functionality is contained in the RU, the RU receives only a baseband signal for transmitted data and does all of the up-conversion and amplification at the RU. This eliminates the need for sending RF signals up to the RU, thus allowing the RU to operate at a higher efficiency than a unit in which the RF signal must travel the length of the pole.
Up-conversion is done in the RU, thus, direct modulation reduces the complexity of the transmit signal line, and provides a significant cost reduction over systems that run a transmit signal up the pole and then up-convert again to RF. Far less RF components are required in the present invention than in the prior art.
Output power calibration can be performed at the factory and the RU can be programmed for usage with any MU. The RU will store full-power settings, as well as reduced power settings, in local memoryxe2x80x94thus enabling cell size adjustment from the RU, instead of at the BTS.
Wilting and blossoming attenuation can be accomplished in the RU rather than in the BTS. Also, output power detection is performed in the RU and, more important, can be used to verify the integrity of the entire signal transit path. Previously, in units where the PA is mounted on the pole, the output power attenuation could be detected, but the operator could not determine if the problem was in the PA module or the MU.
System upgrades can be accomplished more easily as entire RUs or MUs can be replaced. In addition, because like elements are configured together, board or device level upgrades are also more easily accomplished than with traditional BTS units.
These and other advantages of the present invention will become apparent to one of ordinary skill in the art after consideration of the figures and detailed description which follows hereinafter.